This is the answers:
This point is actually covered in the NDC Implementation Guide V2 that was just published.
Indeed, the aggregation function is useful during the initial phase of the shopping conversation – so that the seller/agent can reach out to several airlines in parallel via an aggregator. (see page 14 – An Aggregator’s role is to request Offers from Airlines based on a shopping request initiated by a Seller, and respond to the Seller with a consolidated set of these Offers. This consolidation could involve receiving one or more Offers from Airlines and deciding which Offers and in what form they should be presented to the Seller).
Later on (see page 17), If a Customer chooses to accept an Offer, the Seller will ask the Airline to create an Order. This Order creation request may be sent directly to the Airline, or it may still pass through the Aggregator. If the request passes through the Aggregator, their role is passive and they will act as a proxy, they may continue to aid with authentication of the messages/the actors involved, but they will no longer be applying any routing logic as the Order is placed with the specific Airline that created the chosen Offer originally.